1. Field of the Invention
This invention relates to a photocurable resin, in particular, to an epoxy resin composition curable by cation polymerization induced by photoirradiation.
2. Description of the Related Technology
A cationically photocurable epoxy resin composition which can be cured by cationic polymerization of an epoxy resin induced by photo irradiation, in particular irradiation with light within the UV range, has many advantages; for example, it provides cured products having physical properties comparable with those from a thermosetting composition, it can be less inhibited by oxygene than a radical UV curable composition, and it exhibits a smaller shrinkage than a radical UV curable composition. It has been, therefore, used in various well-known applications such as a liquid crystal display and lamination in a digital video disk.
The resin composition comprises a cationic photoinitiator. It is known that a cationic photoinitiator is usually represented by a salt form, i.e., A+B−, and that a curing rate for the resin composition substantially depends on the type of the anion B−. For example, it is known that curing is quicker for a resin composition comprising an SbF6− containing compound such as triarylsulfonium hexafluoroantimonate or a B(aryl)4− containing compound such as tetrakis(pentafluorophenyl)borate-{methyl-4-phenyl(methyl-1-ethyl)-4-phenyl} iodonium salt as a cationic photoinitiator.
However, our studies have revealed that while an epoxy resin composition comprising such a quickly-curing initiator exhibits good adhesive strength to a non-alkali glass such as a borosilicate glass, it exhibits insufficient adhesive strength to an alkali glass such as a soda-lime glass or a metal such as stainless steel.
On the other hand, it is known that curing is slower for a resin composition comprising a PF6− containing compound such as a triarylsulfonium hexafluorophosphate as a cationic photoinitiator. Our studies have revealed that when used in combination with a quickly-curing initiator, such a slowly-curing cationic photoinitiator improves adhesive strength in adhesion to an alkali glass or a metal to some extent, but not adequately. In some cases, a quickly-curing initiator must be incorporated in order to give a required curing rate for a particular application. It has been, therefore, strongly needed to improve adhesive strength.
Japanese Patent Publication JP-A 6-73159 has described that talc or cordierite is added to a cationically photocurable epoxy resin. The JP '159 publication described that it is essential to use a fluorinated epoxy resin and has listed cordierite powder, talc powder and silica powder as a filler added to a fluorinated bisphenol-A type of epoxy resin. Furthermore, only adhesiveness to BK-7 glass, which does not contain an alkaline metal, has been studied in a test for adhesion to a glass. Thus, the JP '159 publication has disclosed no cationically photocurable resins which provide adhesive strength adequate for adhesion to an alkali glass or to a metal such as stainless steel.